A terahertz wave is an electromagnetic wave that has a component anywhere in a frequency band of 0.03 THz to 30 THz. Characteristic absorptions (absorption of a particular frequency spectrum) resulting from biomolecules as well as from the structures and states of various substances often occur within such a frequency band. By utilizing such properties, inspection technologies through which substances are analyzed and identified in a nondestructive manner are being developed. In addition, the application of these technologies as safer imaging technologies that could replace x-rays and as high-speed communication technologies is desired. This kind of application often utilizes an absorption that is unique to a substance and that is observed as a frequency spectrum. In the case of a configuration of an apparatus that utilizes a frequency spectrum, the quantitativeness of measured frequency spectra is important. This requirement is not limited to the terahertz wave region. With respect to the infrared region, for example, there has been disclosed a technology in which an apparatus is calibrated by calculating a deviation between a theoretical waveform estimated from the optical parameters of a substance used for the calibration and a measurement waveform of the substance used for the calibration (PTL 1).
On the other hand, many THz-TDS apparatuses perform a sampling measurement by using ultrashort pulsed light (hereinafter may be referred to as excitation light) having a pulse width of femtosecond order in order to obtain a waveform having a pulse duration of sub-picosecond order. This sampling of a terahertz wave can be realized by adjusting a time difference between beams of excitation light that reach a generation unit that generates the terahertz wave and a detection unit that detects the terahertz wave, respectively. For example, the time difference is obtained by adjusting the amount of reflection of the excitation light with a stage (may be referred to as a delay optical unit herein) that has a reflection optical system and that is inserted into a propagation path of the excitation light. The accuracy with which a frequency spectrum is measured is influenced by the behavior of this stage. Therefore, there is disclosed a technology that improves the measurement accuracy by properly monitoring the position of the stage and obtaining the exact amount of reflection of the excitation light (PTL 2).